This invention relates generally to harnessing the energy present in the residual magnetic fields present in the armature windings of a generator or motor and using it to power electronic equipment carried by the rotating shaft of the generator or motor. Often it is desirable to mount electronic equipment on the shaft of the rotor of an electrical generator or motor to measure parameters associated with the rotor. This electronic equipment can be used for a variety of diagnostic, data-gathering purposes including, inter alia, ground fault detection and measuring rotor temperature, rotor voltage or rotor current.
Simple electrical generators utilize field windings formed of wire coils mounted on spokes fixed to and positioned radially about the shaft of the generator to define the rotor. These windings have current flowing through them in order to establish a magnetic field. A stationary stator surrounds the field windings of the rotor and includes armature windings. When the shaft is rotated, the magnetic field produced by the field windings interacts with the armature windings, inducing an electrical current in the armature windings, which are electrically connected to the load to which the generator is supplying current. A portion of the armature currents is tapped from the armature windings, rectified into a direct current (DC) and fed back to the field windings of the rotor via slip rings to provide the necessary current to generate the magnetic field. When the generator is initially started, there is no current flowing in the rotor field windings and, therefore, no magnetic field. An initial excitation current must be supplied to the field windings to create an initial magnetic field. This is known as "flashing the field." This initial magnetic field can then interact with the armature windings to induce a current in the armature windings, some of which current is fed back to provide a main source of current to the rotor field windings, replacing the excitation source. Excitation current is often supplied by a smaller, permanent magnet generator (PMG) consisting of permanent magnets mounted on spokes which extend radially from the shaft of the main generator and another set of armature windings in which the excitation current is induced.
Modern, medium to large state of the art generators are more complicated and can, as described below utilize three machines mounted to a common shaft. The first is the main alternator which generates the main output power in its stationary armature windings for use by the end user. The second machine mounted to the shaft is a brushless exciter which supplies the current to the alternator's rotor field windings to create the rotor's magnetic field. The brushless exciter has stationary electromagnetic field windings and rotating armature windings which break the lines of flux of the stationary field, thereby inducing a current in the rotating armature windings which is fed via a rectifier, also mounted on the shaft, to the main alternator's rotor field windings to create the alternator rotor's magnetic field. Use of the brushless exciter on the common shaft obviates the need for slip rings or brushes which are prone to wear and degradation. The third machine attached to the shaft is a standard PMG which has a rotating magnetic field created by permanent magnets which rotate in close proximity to a stationary armature. This permanent magnet generator creates the electrical current needed to create the stationary magnetic field in the stationary windings of the brushless exciter. The current is provided to the exciter via a voltage regulator incorporating a silicon controlled rectifier or other solid state devices.
In operation, electricity is produced in these modern generators by rotating the common shaft of the generator by a motor apparatus such as a gas-turbine engine, diesel engine, or steam turbine thereby causing the moving magnets of the PMG to create a current in the PMG stationary armature. This current passes through a voltage regulator which ensures that appropriate current is fed to the field windings of the brushless exciter in order to maintain a constant generator output voltage. The lines of flux of the exciter's stationary field are broken by the windings of its rotating armature. The alternating current induced in the windings of the exciter's armature is converted to direct current via a rectifier and fed to the rotating field windings of the main alternator, thereby creating a rotating magnetic field. The rotating lines of flux of this field pass over the stationary armature windings of the main alternator, producing electric power which is fed to the end user.
Mounting sensors on the shaft of a generator or motor is known. However, because these sensors are mounted on the spinning shaft of the generator or motor, providing a reliable, steady electrical current to the sensors has presented problems that the prior art methods have failed to adequately overcome. The prior art provides three ways of providing power to shaft-mounted electronic equipment.
One solution the prior art provides is to tap power directly from the field windings. Because the field windings are rotating with the sensors, this solution overcomes the problem of providing an electrical connection between a stationary circuit and a rotating circuit. However, the voltages generated in the field windings typically range from 30 to 2500 volts. Devising a circuit that is able to power a delicate sensor with such a widely varying voltage source can be difficult and expensive. Furthermore, during a fault, the field windings often experience very high voltage transients which can easily damage delicate electronic equipment.
A second solution incorporates a rotary transformer to make the electrical connection between a stationary power source and the rotating circuit feeding the shaft-mounted sensors. Rotary transformers provide the option of tapping power from the armature windings or using independent, reliable power sources. Though using rotary transformers overcomes the problems associated with using power from the field windings, these transformers are expensive, delicate, and difficult to install and align. Furthermore, due to their delicacy, the amount of power that can be drawn through these transformers is limited.
A third solution is to use slip rings in place of rotary transformers. Slip rings are a mechanical means to perform the same function as rotary transformers. They are much cheaper and less delicate than rotary transformers, and are relatively easy to install and align. However, due to their mechanical nature and the high speeds at which generator and motor shafts typically operate, slip rings are prone to wear and can be unreliable. Also, unlike rotary transformers, slip rings do not isolate the rotating circuit from the stationary circuit, leaving the shaft-mounted sensors vulnerable to spikes and surges.
There is a need to provide a power source for shaft-mounted electronic sensors that is reliable, relatively constant, and inexpensive. Preferably this power source should rotate with the generator shaft and should be isolated from the load to which the main alternator is connected.